meaning of sentence

In the medical folklore of the Wild West there was no anesthesia. Heroic cowboys were given a bullet to hold between their teeth and urged to "bite the bullet" instead of uttering unmanly cries of pain.

The depth of the dent mark left in the bullet by the cowboy's teeth supposedly showed the intensity of the pain.

"Bite the bullet" in your excerpt means "face the unpleasantness".

"Kick-start" is a little more technical. I'm under the impression that when one kick-starts an engine, the normal starter is bypassed, and the engine kicks in with a loud VVVVAAAROOOoooom.

Engines are one of my areas of vast ignorance.

what noddy said. I can only add that A kick start is another way of saying that you are using a direct means to most rapidly get your metabloism changing over from a sleep state to a state of full "wide awake".
In the comparison you quoted (it isnt strictly a euphemism),orA kick starter is an old fashioned (Still used on some motorbikes and cycles) means of using a rathcheting spring that directly turnes over a small engine. (A lawn mower with a ratchet wind-up starter is a good parallel). The ring gear is directly driven by the ratchet spring , which is activated by you pumping or "kicking" the foot bar. WIth the engine out of gear it will often start with a roar after a few "kicks". (However, modern key start bikes have taken all the fun out of cranking the bike over)

Some cars, like the beloved Renault DAuphines , had a ratchet starter in case the key start didnt work.(Renault was famous for loading their cars with lots of stuff that didnt work eventually), however, as usual, Ive strayed off the point.

By facing the unpleasant task of exercising in the early morning, you help your metabolism rate to rise quickly and it keeps running higher for hours.

or

To quickly get your metabolism rate up and to keep it running for hours, exercise early in the morning, even though it is unpleasant.

Thanks, guys.